1. Field of the Invention
The invention relates to an information recording medium in which information is optically recorded thereto or reproduced therefrom, an apparatus for forming the information recording medium, an information recording system for optically recording information to the information recording medium, and an information reproducing system for optically reproducing information from the information recording medium.
2. Description of the Related Art
As information recording media in which information is optically recorded thereto or reproduced therefrom, a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) are known. Besides the information read only CD and DVD, a CD and a DVD to which information can be recorded once and a CD and a DVD from which information can be erased and to which information can be recorded again have been developed.
For example, as schematically shown in a cross sectional view of FIG. 11, each information recording medium (hereinafter, referred to as an optical disc) 1 has a laminated structure comprising: a transparent substrate 1c as a relatively thick light transmitting layer having a function of supporting an optical disc main body and a function of transmitting light; a relatively thin protecting plate 1a positioning on the reverse side of the optical disc 1 as compared with the transparent substrate 1c; and a recording layer 1b arranged between the transparent substrate 1c and protecting layer 1a. By irradiating a laser beam from the outside onto the transparent substrate 1c side, information recording to the recording layer 1b by an information recording system or an information reproduction from the recording layer 1b by an information reproducing system is performed.
Research and development of a large capacity optical disc has progressed, and also an information reproducing system for such a optical disc has progressed in which high density recording can be performed on the optical disc.
To realize high density recording, research and development of a material and structure of an optical disc, a manufacturing method thereof, and the like has progressed. On the side of the information recording system and information reproducing system, a method has been considered whereby high density recording and reproduction of information recorded at a high density by a laser beam of a small irradiating diameter are performed by increasing a numerical aperture (NA) of an objective lens for irradiating the laser beam to a recording layer through a transparent substrate.
When optical discs are manufactured, however, sometimes a thickness spot (uneven thickness) occurs on the transparent substrate or, even if a thickness of the whole transparent substrate is even, the thickness is different from the standardized thickness, or the like. If the laser beam is irradiated through the objective lens of a large numerical aperture NA to an optical disc having a transparent substrate with a thickness error or unevenness, spherical aberration easily occurs in the optics for the laser beam because of the thickness error of the transparent substrate. There is such a problem that precision of the information recording and that of the information reproduction cannot be improved.
To properly perform high density recording and reproduction of the information recorded at a high density, it should be considered in various optical characteristics. The characteristics include birefringence of the transparent substrate, pigment sensitivity wavelength dependency and reflectance which are used in the recording layer, power of the laser beam, and the like besides the adverse influence due to the spherical aberration in association with the realization of the high NA of the objective lens. Thus, it is important to suppress the adverse influence which is caused by a variation of those optical characteristics between the optical discs or by unevenness of those optical characteristics existing in each optical disc.
The invention is made to solve the problems and it is an object of the invention to provide an information recording medium, an apparatus for forming the medium, an information recording system, and an information reproducing system, in which a high density recording or a reproduction of information recorded at a high density can be properly performed.
Another object of the invention is to provide an information recording medium, an apparatus for forming the medium, an information recording system, and an information reproducing system, in which the realization of a high NA for performing a high density recording or a reproduction of information recorded at a high density can be promoted.
To accomplish the above objects, according to an aspect of the invention, there is provided an information recording medium which has a light transmitting layer and a recording layer and in which an information recording to the recording layer or an information reproduction from the recording layer by a light which is irradiated from the light transmitting layer side is performed by an information recording system or an information reproducing system, wherein information regarding at least optical characteristics of the light transmitting layer has previously been recorded in the recording layer.
According to the information recording medium with the construction, when the information recording is performed by the information recording system, the information recording system previously optically reads the information regarding the optical characteristics and enables a compensating process for properly performing the information recording to be performed on the basis of the information regarding the read-out optical characteristics. For example, even in the case where a value of NA of an objective lens provided for the information recording system is set to a high value in association with the realization of the high density of the information recording medium, therefore, the optical system having the objective lens is automatically adjusted on the basis of the information of the optical characteristics, thereby enabling the high density recording to the information recording medium to be performed. When the information reproduction is performed by the information reproducing system, the information reproducing system previously optically reads the information regarding the optical characteristics, thereby enabling the compensating process for properly performing the information reproduction to be performed on the basis of the information regarding the read-out optical characteristics. For example, even in the case where a value of NA of an objective lens provided for the information reproducing system is set to a high value in association with the realization of the high density of the information recording medium, the optical system having the objective lens is automatically adjusted on the basis of the information of the optical characteristics, thereby enabling the information recorded on the information recording medium at a high density to be properly reproduced.
According to another aspect of the invention, there is provided an apparatus for forming an information recording medium which has a light transmitting layer and a recording layer and in which an information recording to the recording layer or an information reproduction from the recording layer by a light which is irradiated from the light transmitting layer side is performed by an information recording system or an information reproducing system, comprising: measuring means for measuring optical characteristics of the information recording medium; and recording means for previously recording measurement information measured by the measuring means into the recording layer.
According to the forming apparatus of the information recording medium with the construction, the optical characteristics of the manufactured information recording medium are measured and the information regarding the optical characteristics is previously recorded to the information recording medium. When the information recording to the information recording medium formed as mentioned above is performed by the information recording system, the information recording system previously optically reads the information regarding the optical characteristics and enables the compensating process for properly performing the information recording to be performed on the basis of the information regarding the read-out optical characteristics. For example, even in the case where a value of NA of an objective lens provided for the information recording system is set to a high value in association with the realization of the high density of the information recording medium, an optical system having the objective lens is automatically adjusted on the basis of the information of the optical characteristics, thereby enabling the high density recording to the information recording medium to be performed.
According to the forming apparatus of the information recording medium, when the information reproduction of the formed information recording medium is performed by the information reproducing system, the information reproducing system previously optically reads the information regarding the optical characteristics, thereby enabling the compensating process for properly performing the information recording to be performed on the basis of the information regarding the read-out optical characteristics. For example, even in the case where a value of NA of an objective lens provided for the information reproducing system is set to a high value in association with the realization of the high density of the information recording medium, the optical system having the objective lens is automatically adjusted on the basis of the information of the optical characteristics, thereby enabling the information recorded on the information recording medium at a high density to be properly reproduced.
According to still another aspect of the invention, there is provided an information recording system for recording information to an information recording medium which has a light transmitting layer and a recording layer and in which information regarding at least optical characteristics of the light transmitting layer has previously been recorded in the recording layer, comprising: reading means for optically reading the information regarding the optical characteristics recorded in the recording layer; and optical characteristics compensating means for compensating a deviation of the optical characteristics which is caused when the information is recorded to the information recording medium on the basis of the information regarding the optical characteristics read by the reading means and performing the information recording.
According to the information recording system with the construction, when the information recording is performed, the information regarding the optical characteristics recorded on the information recording medium is optically read, and the compensating process for properly performing the information recording can be performed on the basis of the information regarding the read-out optical characteristics. For example, consequently, even in the case where a value of NA of an objective lens provided for the information reproducing system is set to a high value in association with the realization of the high density of the information recording medium, an optical system having the objective lens is automatically adjusted on the basis of the optical characteristics, thereby enabling the high density recording to the information recording medium to be properly performed.
According to further another aspect of the invention, there is provided an information reproducing system for reproducing information from an information recording medium which has a light transmitting layer and a recording layer and in which the information regarding at least optical characteristics of the light transmitting layer has previously been recorded in the recording layer, comprising: reading means for optically reading the information regarding the optical characteristics recorded in the recording layer; and optical characteristics compensating means for compensating a deviation of the optical characteristics which is caused when the information is reproduced from the information recording medium on the basis of the information regarding the optical characteristics read by the reading means and performing the information reproduction.
According to the information reproducing system with the construction, when the information reproduction is performed, the information regarding the optical characteristics recorded on the information recording medium is optically read, and the compensating process for properly performing the information reproduction can be performed on the basis of information regarding the read-out optical characteristics. For example, consequently, even in the case where a value of NA of an objective lens provided for the information reproducing system is set to a high value in association with the realization of the high density of the information recording medium, the optical system having the objective lens is automatically adjusted on the basis of the optical characteristics, thereby enabling the information to be reproduced from the information recording medium on which the information has been recorded at a high density.